Aparatus for controlling the displacement of an object between any two points

ABSTRACT

Apparatus for displacing an object between any two points along a predetermined path, according to a given law of motion, wherein a signal representing the actual speed of said object is compared with a reference speed signal derived according to the law of motion, and from said comparison an error signal is generated to control the speed of a motor driving such object.

United States Patent 11 1 Brette 1451 Apr, 24, 1973 APARATUS FORCONTROLLING THE [56] References Cited DI PLA E FAN B E T T E KEQ Q IUNITED STATES PATENTS 3,241,015 3/l966 Allen ..3l8/6l7 X [75] Inventor2:3 Franco's Sevres 3,525,883 8/1970 lordanidis...... ..318/681 x3,ll4,869 12/1963 Goodwin 1 ..3l8/6l7 [73] Assignee: Societelndustrielle Honeywell Bull 3,015,767 1/1962 Taylor ..318/617 (SocieteAnonyme), Paris, France 1 Primary ExaminerBenjamin Dobeck [22] FiledJune 1971 Att0rneyRonald T. Reiling et al. [21] Appl.No.: 156,261 [57]ABSTRACT Foreign Application Priority Data Apparatus for displacing anobject between any two June 26 1970 France ..7023798 Points almg@Predetermined Path accmding a given law of motion, wherein a signalrepresenting the 52 us. 01. ..318/56l, 318/617, 318/681 actual Speed ofSaid Object is compared with a 51 Int. Cl. ..G05b 13/00 reference SpeedSignal derived according to the law of [58] Field 61 Search ..318/617,681, 561 motion, and from said comparison an error signal is generatedto control the speed of a motor driving such object.

8 Claims, 5 Drawing Figures Patented April 24,1973

3 Sheets-Sheet 1 v P @qfgg BY ZJ A 1 I A'lT RNEY Patented April 24, 19733,729,668

5 Sheets-Sheet c 59' {I w (0 0) fill) I a Q10 z, 2' i;

(do) Mu rd) (#2) 0/3) INVENTOR AT ORNEY APARATUS FOR CONTROLLING THEDISPLACEMENT OF AN OBJECT BETWEEN ANY TWO POINTS BACKGROUND OF THEINVENTION The present invention relates to apparatus providing fordisplacing an object along a path according to a predetermined law ofdata motion.

Apparatus of this type is utilized in data processing systems to bringread-write heads to a desired track ofa magnetic disc which is adaptedfor storing data.

It is important, in such apparatus, that the object to be displaced bebrought in a minimum time from a starting point to a destination point,that during 'a first portion of the movement the values of accelerationand deceleration be close to the maximum permissible values withoutexceeding them, and that at the end of the movement, before reaching thedestination point, the object assumes a low speed from which it will bebraked so as to stop at the destination point.

It is necessary, further, that this mode ofoperation be achieved despiteanticipated variations of the parameters entering into the equation ofmotion of the movable member.

Apparatus of this type comprising a linear direct current motor is knownin the art. However, such ap-.

paratus demonstrates disadvantages, particularly it lacks stability inoperation, the object displaced does not follow the desired law ofmotion, and such object vibrates at the end of its displacement due tothe strong deceleration to which it is subject during the braking.

It is the object of the present invention to remedy these disadvantages.

SUMMARY OF THE INVENTION According to the instant invention theapparatus providing for displacing an object between any two points of agiven path according to a predetermined law of motion, wherein suchapparatus is provided with a direct current motor, comprises means forapplying to the motor a supply voltage'of which the value is determinedat each moment during at least one portion of the movement by acomparison between the actual speed of the object and a desiredreference speed which the object must have at the position reachedthereby at the moment considered. This reference speed is a function ofthe position of the object in accordance with the above mentioned law ofmotion.

For this purpose, the apparatus of the invention comprises:

In one embodiment of the invention, the signal representing thereference speed is a linear function of the distance remaining to betraveled by the object.

In order to provide for displacing the object both forward and backward,the apparatus comprises means disposed between the function generatorand the comparator and adapted to change, as appropriate, the

polarity of the signal delivered by the function generator.

BRIEF DESCRIPTION OF THE DRAWING The invention will be described withreference to the accompanying drawing, wherein:

a first generator for generating a signal representing the positionreached by the object,

a second generator for generating a signal representing the actual speedofthe object,

a function generator controlled by the signal generated by the firstgenerator for generating a signal representing the reference speed,

a comparator for producing an error signal resulting from comparing thesignal representing the actual speed of the object, generated by thefirst generator, with the signal representing the reference speed,generated by the function generator,

and control means providing for controlling the supply voltage of themotor as a function of the value of the error signal, and particularlyfor controlling the polarity of this supply voltage according to thepolarity of the error signal.

FIG. 1 is a schematic block diagram of apparatus in accordance with theinvention;

FIGS. 2, 3, 4 and 5 illustrate curves demonstrating the operation of theapparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The apparatus of the inventionshown in FIG. 1 comprises a direct current motor M, adapted to advanceby any means, not shown, an object, also not shown, for exampleread-write heads over a magnetic disc of a disc store. Motor M may be alinear direct current motor of the type termed voice coil, whichcomprises a movable winding L that moves linearly when it is energized.The latter is connected in one diagonal of a bridge of switches Il-I4,for example transistors. The other diagonal of this bridge is connectedto the terminals +U and U of a source of direct voltage-The object to bedisplaced is moved in one direction when switches l1 and I4 areconductive, while switches I2 and I3 are open, and in the otherdirection when switches l2 and I3 are conductive, while switches I1 andI4 are open.

The apparatus of the invention further comprises a digital register Rcontinuously delivering at its output terminal a digital signalrepresenting the distance d remaining to be traveled by the object toreach its destination p'oint (distance from destination register).Register R comprises an input terminal 1 providing a representation ofthe initial distance d which separates the object from the destinationpoint and an input terminal 2 on which is received a signal representingthe distance x traveled .by the object. This x signal is furnished by ameasuring device Me, coupled mechanically to the movable memberof motorM. Register R functions as a distance reducer, subtracting from thevalue d the actual distance x traveled. At each instant register Rsupplies the value corresponding to d d i x. The digital output signalof register R is converted to analog form by a digital-analog converterC and controlled, in turn, a function generator G which delivers anelectrical output signal corresponding to each value of d, such outputsignal representing a predetermined theoretical value V, of the velocityof the object. This value V becomes constant, equal to the low velocityV when the distance d becomes less than a given value; i.e., when theobject is in the vicinity of its destination point. The relationship V=f(d) between V and (I may be linear. Function generator G may be of anyknown type, for example, it may consist of a network of resistors.

The output terminal of generator G is connected to a logical element L0,both directly and through an inverter I. Logical element L0 iscontrolled from two terminals 3 and 4. Thus, logical element Lo receivesboth the signal V and the signal V but delivers at its output terminalonly one of such signals, as a function of the commands received onterminals 3 and 4. Terminals 3 and 4 enable the selection of either aforward movement (MAV) or a backward movement (MAR) 'for the object.

The output signal of logical element Lo has a value corresponding to adesired speed for the object and a polarity corresponding to therequired direction of displacement of the object. This output signal oflogical element L is compared by a comparator Co with an electricalsignal Vr, of which the amplitude and polarity respectively representthe actual speed and direction of displacement of the object. The signalVr is furnished by any appropriate device T, which may be a tachometercoupled mechanically to the moveable member of motor M.

The speed error signal e delivered by comparator C0 (algebraicdifference between the signals V and Vr) is delivered to the inputterminal of an amplifier A. Ac cording to the polarity of error signale, amplifier A simultaneously controls either switches II and I4 orswitches I2 and I3.

For explaining the operation of this apparatus, it will be assumed, thedirection of displacement ofthe object having been chosen, that thefunction V,, f(d) is linearly decreasing from a value f (d) for thedistance d to travel, at time t to the value f (d) Vb for the distance11 remaining to travel, at time 1 (see FIG. 4) Consequently, V is anexponential decreasing function g(t) of time t (see FIG. 3) from thevalue g,,(t) at time t to the value g (t) Vb, at time t In accordancewith the invention, the function ,V,, =f(d) is chosen in the followingmanner.

If the control apparatus of the invention were non existent; i.e., ifmotor M operated with no load and if moveable winding L of motor M wassupplied with square voltage pulses C1 and C2 of opposite polarities(the first for acceleration and the second for breaking), the speed V ofthe movable member of the motor will have the following behavior (seeFIG. 2). During the duration t --t of the first 2 pulse C1, the speedincreases from O (for t and d to a value V1 (for :1 and d1) along anexponential increasing curve 0-Vl, which may be very close to a straightline for a suitable choice of the time constant of the circuit.Conversely during the duration t t, of the second square pulse C2, the.speed decreases from V (for t and 11,) to V 0 (for t and d along anexponential decreasing curve Vl-V2, which can also be close to astraight line.

Thus, in the no load response to a supply of inverse square pulses, thespeed taken by the moveable member of motor M has the behavior of atriangle O-VI-VZ, which is isosceles, if a favorable choice is made fort r t, t the amplitude of the square pulses being equal.

If, as shown in FIG. 4, this no load velocity is represented as afunction of the distance remaining to travel, the linear portions 0-VIand Vl-V2 are transformed into two portions O-Vl and V'lV2, which areequal parabolas having their vertices at d and at and the axis 0d as anaxis of symmetry these two portions intersect at V'l (for d and ttherefore the function V =f(d), to be generated by function generator G,

is selected in a manner to be an approximation of at least a part of theportion of parabola V'l-V'2, which is very different from the parabolaportion 0V'1. For this purpose, the above mentioned straight Iinef (d) 5f (d) can be selected it is apparent that a more complicated functionVd=f(d) can be chosen, particularly a broken line enveloping the portionof parabola V'lV 2 (FIG. 5) in a manner to enable operation of the motoras long as possible in a manner approaching at the no load condition.

For d and t Vd takes the value of the constant speed V,, to which it isdesired to reach.

These may be obtained in various ways; i.e., by utilizing a functiongenerator G capable of delivering at its output terminal a signal towhich a value V =V corresponds to any value of (1 less than d The sameresult may also be obtained by utilizing a register R which decrementsto a residual data value such that the function generator G, controlledby the corresponding output signal furnished by converter C, delivers atits output terminal a signal representing the velocity V Referring nowto FIG. 3, the straight line V F(d) is transformed to an exponentialdecreasing function g (t) g (t),joining at g (t) the constant velocity VThus if a representation of the distance d to travel is applied to inputterminal d of register R, Vd is initially large, where as Vr is smalland therefore, the error voltage e is positive as (1 decreases (or tincreases), Vd decreases and Vr increases, so that e decreases butremains positive. The speed Vr of the movable member increases thereforeaccording to the curve 0-V 1 of FIG. 3. At time T1, the curve 0-V lmeets the curve Vd=g(t) at t V l, and therefore the polarity of theerror voltage e reverses consequently, the switches which had beenconducting open, whereas those which had been open close. The voltage atthe terminals of movable winding L is reversed and produces braking ofthe moveable member.-The speed Vr thereupon decreases. At time T,,between t and t Vr becomes mV,,. The error voltage e reverses again,which determines a new reversal of the voltage at the terminals ofwinding L. Thus, as a consequence of the succesive reversal of thevoltage of the terminals of winding L, the speed V, of the moveablemember takes on values alternatively greater less than the referencespeed Vb and reaches a value approximately (3 (2) at time :2. At thismoment the value of the signal Vd imposes the velocity Vb on themoveable member.

FIG. 4 similarly represents the variations of the speed of the moveablemember of the function of distance d.

If the law Vd =f(d) is selected so that, on the one hand, it correspondsto a law of deceleration as an optimum function of time and on the otherhand, it is an approximation of the law of speed as a function of timecorresponding to the most unfavorable conditions of operation (minimumsupply voltage, maximum friction), in other conditions of operation thedeviceoperates in a manner to impose this law of speed on the moveablemember, thereby providing the benefits of the law of deceleration withthe same performances as under the most unfavorable conditions. 7

Electronic switches are most favored for utilization as switches I1, I2,I3, and I4. These can be made to operate in the linear region in amanner to vary linearly the voltage at the terminals of the moveablewinding as a function of the error voltage e, at least for small valuesof this error voltage.

The apparatus of the invention provides therefore for assuring that themoveable member, after a rapid displacement in accordance with a givenlaw of movement, reaches a desired speed V,, at any moment greater than1 or, which is similar, for any distance d remaining to travel which isless than d The apparatus of the invention may similarly be utilized fordisplacing magnetic heads at a constant speed over a given distance, forexample in the course of operations which consist of displacing magneticheads from their rest position outside the disk to the operatingposition above the disk tracks, and the reverse.

This movement at constant velocity is obtained by applying a fixedsignal to the input terminal of register R and by suppressing thedecrementing operation in register R during the movement of the heads;for example, by suppressing by means of a switch J the connectionbetween the measuring device Me and input terminal 2 of register R.

Various embodiments of the invention may be realized; by, thecombination of a digital-decrementer and digital-analog converter may bereplaced by a linear potentiometer driven by the movement of themoveable member.

I claim:

1. Apparatus for controlling the displacement of an object between anytwo points wherein a direct current motor is provided comprising: 1

a first generator means for generating a'first signal representative ofthe distance remaining to be traveled by said object, said firstgenerator means comprising: t I means for receiving a signalrepresentative of the distance which said object must travel between Isaid two points, v means for measuring the current position of saidobject and for providing a signalrepresentative of the current position,v a distance reducer means for reducing the signal representative of thedistance which said object must travel by the amount of distancetraveled by said object, said distance reducer means comprising adigital register followed by a digital to analog converter;

a second generator means for generating a second signal representativeof the actual speed of said object;

a function generator, controlled by the first signal representative ofthe distance remaining to be traveled by said object, for generating areference speed signal;

means for comparing the second signal representative of the actual speedof said object with the reference signal and for generating an errorsignal indicative of the signal difference; and 7 control means forcontrolling the polarity of the supply voltage to said direct currentmotor in accordance with the polarity of said error signal.

2. The apparatus of claim 1 wherein said distance reducer furthercomprises means for storing a minimum distance remaining to be traveledby said object so that the representation of the distance remaining tobe traveled by said object is reduced'until it reaches the minimumdistanceremaining to be traveled by said object.

3. The apparatus of claim 1 wherein said first generator meanscomprises:

means for surpassing the signal representative of the current position.

4. The apparatus of claim 1 further comprising:

4 an inverter means, interposed between said function generator and saidcomparing means for inverting the signal from said function generator.

5. The apparatus of claim 2 wherein the signal generated by saidfunction generator is a straight line segment when said distance reducermeans is reducing the distance remaining to be traveled by said objectto said minimum distance remaining to be traveled by said object.

6. The apparatus of claim 5 wherein the signal generated by saidfunction generator is a broken line when said distance reducer means hasreduced the distance to be traveled bysaid object to the minimumdistance remaining to be traveled by said object.

7 The apparatus of claim 1, wherein the signal generatedby said functiongenerator is a linear function of the distance remaining to betraveledby said objects.

, 8. The apparatus of claim 1, wherein said direct current motor is ofthe type comprising a linearly moving winding, said winding beingmounted in a bridge of switches controlled simultaneously across adiagonal by the signal resulting from the comparison between the actualspeed signal and the reference speed signal.

1. Apparatus for controlling the displacement of an object between anytwo points wherein a direct current motor is provided comprising: afirst generator means for generating a first signal representative ofthe distance remaining to be traveled by said object, said firstgenerator means comprising: means for receiving a signal representativeof the distance which said object must travel between said two points,means for measuring the current position of said object and forproviding a signal representative of the current position, a distancereducer means for reducing the signal representative of the distancewhich said object must travel by the amount of distance traveled by saidobject, said distance reducer means comprising a digital registerfollowed by a digital to analog converter; a second generator means forgenerating a second signal representative of the actual speed of saidobject; a function generator, controlled by the first signalrepresentative of the distance remaining to be traveled by said object,for generating a reference speed signal; means for comparing the secondsignal representative of the actual speed of said object with thereference signal and for generating an error signal indicative of thesignal difference; and control means for controlling the polarity of thesupply voltage to said direct current motor in accordance with thepolarity of said error signal.
 2. The apparatus of claim 1 wherein saiddistance reducer further comprises means for storing a minimum distanceremaining to be traveled by said object so that the representation ofthe distance remaining to be traveled by said object is reduced until itreaches the minimum distance remaining to be traveled by said object. 3.The apparatus of claim 1 wherein said first generator means comprises:means for surpassing the signal representative of the current position.4. The apparatus of claim 1 further comprising: an inverter means,interposed between said function generator and said comparing means forinverting the signal from said function generator.
 5. The apparatus ofclaim 2 wherein the signal generated by said function generator is astraight line segment when said distance reducer means is reducing thedistance remaining to be traveled by said object to said minimumdistance remaining to be traveled by said object.
 6. The apparatus ofclaim 5 wherein the signal generated by said function generator is abroken line when said distance reducer means has reduced the distance tobe traveled by said object to the minimum distance remaining to betraveled by said object.
 7. The apparatus of claim 1, wherein the signalgenerated by said function generator is a linear function of thedistance remaining to be traveled by said objects.
 8. The apparatus ofclaim 1, wherein said direct current motor is of the type comprising alinearly moving winding, said winding being mounted in a bridge ofswitches controlled simultaneously across a diagonal by the signalresulting from the comparison between the actual speed signal and thereference speed signal.